Revert "Use __builtin_clz instead of default implementation of fixnormz on GCC"

This reverts commit e036dbc8.

Upstream took __builtin_clz in use (in the new header
libFDK/include/x86/clz_x86.h, together with an alternative
implementation for MSVC), so this change can be dropped to
minimize the differences.

libFDK/include/clz.h

@@ -118,11 +118,6 @@ inline INT fixnormz_S (SHORT a)
 {
   return fixnormz_D((INT)(a));
 }
-#elif defined(__GNUC__)
-inline INT fixnormz_S (SHORT a)
-{
-    return a ? __builtin_clz(a) : 16;
-}
 #else
 inline INT fixnormz_S (SHORT a)
 {
@@ -139,12 +134,6 @@ inline INT fixnormz_S (SHORT a)
 #endif
 
 #if !defined(FUNCTION_fixnormz_D)
-#if defined(__GNUC__)
-inline INT fixnormz_D (LONG a)
-{
-    return a ? __builtin_clz(a) : 32;
-}
-#else
 inline INT fixnormz_D (LONG a)
 {
     INT leadingBits = 0;
@@ -157,7 +146,6 @@ inline INT fixnormz_D (LONG a)
     return (leadingBits);
 }
 #endif
-#endif
 
 
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